1. Field of the Disclosure
The present disclosure relates to a photoacceptive layer comprising a metal oxide of a core-shell structure and a solar cell using the same, and more particularly, to a photoacceptive layer comprising a metal oxide of a core-shell structure which can improve photoconversion efficiency by improving a electron migration path, and a solar cell using the same.
2. Description of the Related Art
To address currently occurring energy problems, studies on alternatives to conventional fossil fuels have been conducted. In particular, broad studies on the utilization of natural energy such as wind power, atomic energy, and solar power for replacing petroleum resources which may be depleted within tens of years have been conducted. Among these, a solar cell using solar energy is unlimited in its resources and is environmental friendly unlike other energy sources. Selenium (Se) solar cells were initially developed in 1883 (see http://en.wikipedia.org/wiki/Solar_cell) and Silicon (Si) solar cells have more recently been considered.
However, such a Si solar cell is limited in practical use and cell efficiency due to its high manufacturing costs. For this reason, the development of a dye-sensitized solar cell that can be very inexpensively manufactured has been actively considered.
Unlike a Si solar cell, a dye-sensitized solar cell is a photoelectrochemical solar cell comprising as main constituents photosensitive dye molecules, which can absorb visible rays to produce electron-hole pairs, and a transition metal oxide which transfers the produced electrons. A representative example of currently available solar cells was reported by Gratzel et al. of Switzerland in 1991. The solar cell of Gratzel et al. includes a semiconductor electrode composed of titanium dioxide (TiO2) nanoparticles which are covered with dye molecules, a counter electrode (Pt electrode), and an electrolyte interposed therebetween. This cell is in the attracting interest due to its low manufacturing costs per power unit compared to a conventional Si solar cell.
FIG. 1 illustrates a structure of a dye-sensitized solar cell. Referring to FIG. 1, the dye-sensitized solar cell includes a semiconductor electrode 10, an electrolyte layer 13, and a counter electrode 14. The semiconductor electrode 10 consists of a transparent conductive substrate 11 and a photoreceptive layer 12. That is, the electrolyte layer 13 is interposed between the semiconductor electrode 10 and the counter electrode 14.
The photoreceptive layer 12 is generally composed of a metal oxide 12a and a dye 12b. The dye 12b can be represented by S, S* and S+ which respectively designate neutral, a transition state, and an ionic state. When the dye 12b absorbs sunlight, electron transition from a ground state (S/S+) to an excited state (S*/S+) occurs to produce an electron-hole pair. The excited electrons (e−) are injected into a conduction band (CB) of the metal oxide 12a to produce an electromotive force.
All the excited electrons do not migrate to the conduction band of the metal oxide 12a and recombine with dye molecules to return to the ground state or induce a recombination reaction in which electrons that migrate to the conduction band combine with redox couples in the electrolyte 13. This causes the photoconversion efficiency to decrease, resulting in a reduction in electromotive force.
To prevent the recombination reaction, attempts to form a protective layer on the metal oxide 12a have been carried out in which the metal oxide 12a has a core-shell structure. However, the largest problem in this instance is that since different oxides are used to form the core-shell structure, interfacial resistance increases, which makes it difficult for electrons to migrate to a central core through an outer shell. That is, since a conventional oxide layer includes core particles having a protective layer (shell) formed thereon as illustrated in FIG. 2, electrons produced from dye molecules must migrate to a transparent conductive substrate through a shell, a core, a shell, a core, etc., and thus electron migration is difficult due to interfacial resistance generated when electrons migrate through interfaces.
Moreover, the oxide used to form the protective layer (shell) has a lower resistance than the core particles, and thus has an increased reactivity to the electrolyte layer. As a result, the protective layer cannot prevent the recombination reaction.